I find the documentation to be ambiguous. Or perhaps it is just that I am obtuse. Anyway, I decided to take the long path. I uninstalled 3 clients, upgraded on the server to 2015, 2016, 2017, and 2018, then installed 3 clients. It all worked. @MichaelMcDonald, the total process took 2 hours. Thanks to everyone for your replies!